In color display devices employed for image display on computers or TVs, a plasma display unit having a plasma display panel (hereinafter referred to as a PDP) has recently received considerable attention as a color display device with large sized screen but lightweight body due to its low-profile structure.
A PDP displays image in full color by performing an additive color process on red, green, and blue—known as the three primary colors. To realize the full color display, a PDP has phosphor layers that are respectively prepared for emitting red (R), green (G), and blue (B) of the three primary colors. A phosphor layer is formed of phosphor particles. The phosphor particles are excited by ultraviolet rays generated in discharge cells in the PDP, so that visible lights of red, green, and blue are produced.
As the chemical compounds typically used for the phosphors above are, for example, Zn2SiO4:Mn2+, which is a green emitter with a tendency to be negatively (−) charged; BaMgAl10O17:Eu2+, which is a blue emitter with a tendency to be positively (+) charged; and (Y,Gd) BO3:Eu3+, Y2O3:Eu3+, which are red emitters with a tendency to be positively (+) charged (for example, see O plus E, No. 195, pp. 98-100, February 1996).
Each phosphor is manufactured through solid phase reaction—after mixed predetermined material, the mixture is baked at high temperature beyond 1000° C. (for example, see Phosphor Handbook, pp. 219-220, Ohm-sha). Because the baking process sinters the phosphor particles, the phosphor particles are crushed to eliminate clotted particles, but are crushed lightly so as not to break the crystallized structure that invites poor luminance. After crushing, the phosphor particles are classified to obtain an average particle diameter for each phosphor particle: preferably, 2-5 μm for the red, and the green phosphors, 3-10 μm for the blue phosphor. The reason why the phosphor particles should be lightly crushed and classified is described below. To form a phosphor layer of a PDP, manufacturers have conventionally employed a screen printing method in which the phosphor particles of each color are processed into paste and the paste is applied by screen printing; and an inkjet applying method in which paste-like phosphor particles are applied with a nozzle (that is introduced in, for example, Japanese Patent Unexamined Publication No. H06-273425). The light crushing and classification can eliminate clotted particles that can cause an uneven application of the phosphor paste or a clogged nozzle in the phosphor paste applying process.
That is, the classified phosphor particles after experienced light crushing offer a uniform particle diameter and particle size distribution, whereby a smooth surface without irregularities can be expected. In forming a phosphor layer, the phosphor particles having smaller, closer to uniformity in size of a particle diameter and closer to a sphere in shape can offer a smoother coating surface. Such desirable particles improve filling density in a phosphor layer; accordingly, increasing luminance efficiency by virtue of increase in emitting surface area of phosphor particles. The advantages above contribute to stable operations in driving a PDP.
On the other hand, a phosphor is an insulant, which is basically formed of a crystal that is stoichiometrically produced from various kinds of elements. The chemical bond of the crystal itself is the ionic bond rather than the covalent bond. A phosphor exhibits different charge characteristics according to electronegativity and the crystal structure of the elements forming the phosphor. Some suggestions about stabilizing the charge characteristics of phosphors have disclosed (see, for example, Japanese Patent Unexamined Publication No. H11-86735, Japanese Patent Unexamined Publication No. 2001-236893, and Japanese Patent Unexamined Publication No. 2002-93321).
A PDP employing the combination of conventional phosphor material has problems below, which are caused by the charge characteristics of each phosphor particle.
Specifically, a PDP employing the conventional combination of Zn2SiO4:Mn2+ for green; BaMgAl10O17:Eu2+ for blue; and (Y,Gd) BO3:Eu3+ or Y2O3:Eu2+ for red has a pending problem below. Of the phosphors employed above, the surfaces of the blue and the red phosphor particles bear positive (+) charge, having an amount of charge of ranging from +1.2 μC/g to +1.1 μC/g measured by a blow-off charge measuring method, which is a widely used method for measuring an amount of charge of powders). On the other hand, the surface of the green phosphor particle of Zn2SiO4:Mn2+ bears negative (−) charge, having an amount of charge of −1.5 μC/g measured by the same method. The reason why the surface of the green phosphor particle bears negative charge results from the fact below. Compared to the stoichiometric ratio of zinc oxide (ZnO) to silicon oxide (SiO2) of 2 to 1, the green phosphor of Zn2SiO4:Mn2+ in the practical use contains the amount of SiO2 has greater than the stoichiometrically determined amount, having a mixture ratio ZnO to SiO2 of 1.5 to 1. The crystal of Zn2SiO4:Mn2+ contains excessive SiO2 on the surface, and SiO2 is likely to bear negative (−) charge due to a great electronegativity on its physical properties.
Generally, in a PDP having a phosphor layer in which a negatively (−) charged phosphor and a positively (+) charged phosphor coexist, the difference in the charge characteristics introduces variations in the amount of discharge through the repeated PDP driving operations. Due to the variations in the charge characteristics, a PDP can't keep a consistent voltage of address discharge on the application of voltage for display, resulting in discharge failure, such as variations in discharge and no discharge.
The difference in charge characteristics has also a problem in forming the phosphor layer using an inkjet applying method. In the inkjet method, phosphor ink is continuously fed through a narrow nozzle to a barrier rib on the rear substrate. The phosphor ink, which is positively (+) or negatively (−) charged due to the friction caused at ink-jetting, is often launched with a bend, and therefore, the ink cannot be evenly applied on the surface of the barrier ribs. In particular, employing each of the three phosphor ink having difference in charge characteristics makes charge control of the rear substrate difficult, thereby inviting the uneven application of the phosphor ink that spoils the view on the PDP.
In driving a PDP, a 147 nm-ultraviolet ray, which is a resonance line with respect to xenon (Xe), is employed for the excitation source of emission. Because of such a short wavelength and therefore poor permeability of the ultraviolet rays, the excitation occurs at only the surface area of the phosphor. That is, the surface condition of the phosphor particles is the most susceptible to luminance degradation. The phosphor surfaces bearing positive or negative charge tell that many dangling bonds occur on the surface of the phosphor particles. Such a surface condition easily captures impurity gases including a hydrocarbon-based gas generated in a PDP. The captured impurity gases are decomposed by plasmatic activity in the PDP to create active hydrogen (proton), by which the surface of the phosphor is reduced to non-crystalloid. This is the main factor that leads to luminance degradation. Besides, in aging or driving a PDP, the phosphor surface bearing charge encourages a collision between positive (+) ions, such as Ne+, Xe+, H+, or between negative (−) ions, such as CHXn− (hydrocarbon-based gas), O2−, in discharge plasma, thereby causing crystal destruction. The phosphor surface bearing charge, as described above, can lead to a fatal degradation including luminance degradation of a PDP.
The present invention addresses the problems above. It is therefore the object of the invention to control the amount of charge of the phosphors so that the absolute value of the amount of charge of each phosphor—green, blue, and red—is determined to be at most 0.01 μC/g, preferably, to be zero.